Method of transmitting secret messages



Aug. 13, 1940. P.- KOTOWSKI EI'AL METHOD-OF- TRANSMII'TING SECRETMESSAGES Filed May 6, 1936 INVENTOR; Auh Kowgmsm BY m M ATTORNEY Theinvention will Patented Aug. 13, 1940 UNITED STATES PATENT oFFicE;

2,211,132 METHOD or 'mansm'r'rmc SECRET MESSAGES 7 Paul Kotowski andKurt Dannehl, Berlin, Germany, assignorsto Telefnnken Gesellschaft fiirDrahtlose Telegraphic m. b. H., Berlin, Germany, a corporation ofGermany Application May 6, 1936, Serial No. 78,184

In Germany May 9, 1935 1 Claim. (c1. Pill-1.5)

magnetic sound reproduction of sound pictures or disks.

best be understood by referring to the accompanying drawing, in which:

Fig. l is a curve showing the proportional relationship between thespeech disturbing oscillations known in the prior art;

: Fig. 2 is a curve showing the proportional relationship between thespeech and disturbing oscillations of this invention;

- Fig. 3 is a schematic detail of a rotating generator which producesthe disturbance noise of this invention.

Methods of secret telephony are known in the prior art in which throughthe superposition of any disturbing "energies on the speech energy coverup the speech and cause what may be called a ciphering. Methods of thistype have not become popular especially in the field of wirelesstelephony, however, since according to the requirements demanded of the"ciphering," the disturbance voltage must be three to fiVe timesthemaximum, speech voltage so that the useful modulation of thetransmitters becomes thereby so low that the transmitter power must beconsiderably increased in order to obtain the same range as in the caseof undisturbed speech (an 40 increase ten to twenty five times). This isclearly shown by Fig. 1, in which curve-ST indicates the disturbancevoltage and. SP the speech voltage.

The invention is based upon the teaching that a disturbance analogous tothe noise voltages of talking films, record plates or tubes, is sumcientto obscure the speech to apoint of complete unintelligibility already atamplitudes which are. 1

1.2 to 1.3 times larger than the maximum speech amplitudes. an idealdisturbing band for which a. relatively small amplitude is suiiicient inorder to render the speech oscillations undecipherabie to a suitabledegree. Where the ideal disturbance spectwin is not fully attained, alarger disturbing amplitude is required. A generator for such noise Sucha film, or disk noise has thusvoltage is suitably such as to provide afrequency spectrum of approximately the same strength throughout thelimits between the zero frequency and the highest frequency used, and isshown by curve a in Fi 2. Also, in this figure, ST represents thedisturbance voltage and SP the speech voltage. Smaller generators ofdisturbance and disturbance lines will be adequate if a narrowerfrequency band underlies the calculation of the disturbance generator,for instance in accordance with the curve b of Fig. 2. In thiscase, the

disturbance is distributed over the frequency band in approximately thesame manner as the speech energy. It has then suitably a slightlypronounced maximum'between 500 and 1000 cycles. The disturbance producermay be designed as rotating generator for instance in that a rotatabledisk S shown in Fig. 3 of irregular shape and having slots or pointedprojections, produces short pulses in a corresponding scanning organ ATthrough electrical, mechanical or optic means, whereby the duration ofan individual pulse is approximately equal or shorter than the halfcycle of the highest disturbance frequency used. When narrowing the bandof the disturbance frequency, then the irregularly distributedindividual pulses are-so dimensioned that the majority thereof have aduration from 0.2 to 0.8 miiii-seconds. Such a disturbance generatorcould for instance also be obtained in that a disturbing noise, known tobe suflicient (record,

plate, talkingfilm, water fall, etc.) is transformed into fluctuationsof light, and these fluctuations are photographed on a rotating plate.In like manner, an endless film band or record plate with endlessgrooves could be utilized. As compared with a round disk withirregularly pointed projections provided mechanically thereon, thismethod has the advantage that the disturbance patterns producedtherewithcan be easily produced at the receiver through a copying methodin any desired number for the purpose of deciphering. The parts ofmechanical disturbance generators by which the frequency is determinedcan be produred in simultaneous ways' for instance by means of machininga plurality of discs mounted on a single mandrel.

In order/to emphasize the rotation frequency of the disturbancegenerator in the mixture of the disturbing frequencies and which isofpracti'cal usefulness for many purposes, a mark K (in Fig.3) is providedat a certain place on disc :8. (Pause in' the disturbing noise, ordefinite Under certain conditions, instead of extending the uniformdisturbance spectrum from zero to the highest frequency as shown bycurve SI of Fig. 2, it may be desirable to filter out by electricalfilters totally or partially the range of for instance 200-2500 cyclesat speech transmission. The particular advantage of the method describedby this invention resides in that the transmitter canbe more effectivelymodulated for the benefit of the speech, in other words at equal speechstrength, a-smaller powered transmitter can be used. e

The ciphering can be provided either by superposing by way oflow-frequency the disturbing noise directly. on the speech, or else bymodulating an auxiliary high-frequency with the disturbing noise andthereafter superposing this auxiliary frequency on the actual carrierfrequency. 1

At the receiver side the deciphering is carried out accordingly.By-imeans of an identical disturbance generator operating in synohronismand in equal phase with that at the transmitter side, the alternatingdisturbance current having the proper phase position and amplitude issuperposed on the arriving speech having been -ciphered by thedisturbance noise, so that irregular alternating current, saidalternating current forming the disturbance noise having a frequencyspectrum which covers substantially the entire speech frequencyspectrum, the amplitude of the frequency spectrum eicceeding butslightly the amplitude of the message frequencies, transmitting thecombined message and disturbance noise on a high frequency carrier,receiving said combined message and disturbance noise'in a proper phaseposition with respect to the phase of the combined transmitted messageto make said disturbance noise disappear, and removing 20

